Frequently Asked Questions

For a full list of Frequently Asked Quenstions regarding the Hydrelio© system, please download our full list here

1. HYDRELIO FLOATS

1.1 COMPOSITION

HYDRELIO floats are made of HDPE, and connection pins are made of polypropylene combined with glass fibre. The manufacturing process is blow moulding for the floats, and injection for the pins.
Several additives are added to the raw material, such as UV protection and colouring agents.

1.2 UV PROTECTION

There are additives in the HDPE of floats, including UV protection agents. It is the same dose for any manufacturing batch, but based on this data we can easily calculate how long the floats will last with the protection of the UV agent. It is linked to the solar radiation of the location of the project.

Usually, it takes between 15 and 20 years before the UV starts to slowly damage the floats. Then it takes 5 to 10 more years before the floats start to be so damaged by the UV they need to be replaced.

Additionally, only a few parts of the floats are exposed to light.

A more detailed calculation can be provided regarding the site.

1.3 METALLIC PIECES – CORROSION

Regarding the fixing system, the main parts are made of aluminium 6060, an alloy designed for usage in contact with food. It has very good corrosion resistance. We can provide either anodised or non-anodised pieces.

The fastening parts are in inox A4 which is mainly used in food industry due to its better corrosion resistance capacity.

1.4 SEAL LINE

Whilst looking at the floats you may note a kind of seal line all around the float. However it is not a seal line: the float is manufactured in one shot. The line appears because the mould is composed of 2 pieces, but the float is blow moulded inside in one shot, therefore there is no particular weakest point there.

1.5 TENSILE AND DYNAMIC BINDING TESTS

HYDRELIO floats have been tested through tensile tests and dynamic binding tests (fatigue).

For the specification of the dynamic bending test, we considered the passage of a storm during 2 hours with wave’s period equal to 2 sec, which would happen four times in a year during the operating life of the plant (25 years). It corresponds to 360000 cycles with an angular movement between two adjacent floats of maximum 15°.

In order to compare to operation conditions, a swell with a wavelength equal to 6 meters and a wave height equal to 1 meter creates a maximum angle of 15° between two adjacent floats.

Compared to simple tensile test without dynamic bending before, we noticed that tensile strength of the floats had decreased of only 6 % with dynamic bending.

1.6 CONNECTION PIN/ EARS STRENGTH

Connection pins and ears have been tested through tensile and dynamic binding tests.

The tests revealed that the failure appears on the ears, and not on the connection pins. Therefore the connection pins are stronger than the ears.

Connection pins have been tested separately and can withstand a load up to 2000 daN. However, the maximum load to consider for the ears is 600 daN.

1.7 SOLAR ISLAND DESIGN

The design of the solar islands is based on the requested electrical design details, such as the number of PV panels per string (depending on the inverter specifications).

Ciel & Terre usually design the solar island to have 1 string = 1 row of PV panels, in order to avoid mutual shadings of PV panels within one same string.

The number of strings on the solar island is calculated regarding the selected inverter specifications. Usually Ciel & Terre design the strings configuration using junction boxes and main boxes to collect the string cables on the solar island. This means there are only few submarine cables connecting the solar islands to the inverters. In this case, the junction boxes and/or main boxes are helpful, but otherwise Ciel & Terre designs the string configuration based on usual boxes specifications.

1.8 EXTRA FLOATS

Some extra floats, free of PV panels, are needed inside and outside the solar island:

  • They are needed inside the solar island to have space to install the junction or main boxes
  • They are need outside the solar island (the additional ring of floats) to improve buoyancy of the solar island.

An additional ring is needed because of the wind loads.

If anchoring at the bottom is selected, when wind is at maximum, the loads spread in the mooring lines pulling down the solar island into the water. To make sure that PV panels will never be in contact with water, and regarding the volume of the floats, an extra ring is needed.

If anchoring on the banks is selected, the extra ring is needed for the case when water level is at its lowest, to prevent mooring cables to scrape the PV panels.

1.9 DUOPITCH DESIGN

The usual configuration of the solar island is rows of PV panels oriented the same direction, with the alternation of rows of PV panels and rows of secondary floats for maintenance path.

But in specific cases a duopitch configuration can be proposed, in order to increase the installed capacity for a given usable area. In this case it is recommended to design with a back to back shape “/\”, in order to lower the wind loads. The “V” shape would contrariwise increase the wind loads.

1.10 MAINTENANCE PATH WITH GAPS

On request of the client we can design the solar island with maintenance path with gaps, in order to lower the number of secondary floats needed. This way the solar island design is more cost-effective regarding both the number of floats and the shipment (less containers required).

This way the maintenance path, composed of secondary floats, have gaps. But it should not been considered as an issue for the O&M company. In this case, few free secondary floats are provided, and can be placed upon the gaps by the O&M team where they need to operate, and remove when they leave.

Okegawa PV plant has been designed this way as we can see on the pictures below:

2. FIXING SYSTEM

2.1 DC BOXES SUPPORT

To install these boxes, we use 4 aluminum rails on one free main float (the same rails than PV panels fixing rail). Those rails are used to hold an aluminum plate instead of a PV panel. On this plate, we use screws to fix the junction/main box.

On the below photo, we can see the junction box support ready to have a junction box installed on it.

3. ELECTRICAL DESIGN

3.1 EARTHING

Each PV panel of each solar island shall be grounded. The earth shall match the conditions defined in local standards and norms.

Usually in the project developed in France, PV panel are connected to the junction box earth via earth cables. Then there are many solutions, depending on the quality of the earth on site:

  • - Earth cable of each junction box hangs into the water, which sometimes provides a sufficient earth for admissible earthing.
  • - Earth cable of each junction box is connected to the anchors at the bottom of the lake
  • - Earth cable of each junction box is connected to the banks (ground)

3.2 CABLE TRAYS OR SHEATH

3.2.1 ON THE SOLAR ISLAND

On the solar island, string cables (from PV panels to junction box) and cables from junction boxes to main boxes can be placed into sheaths. It enables to guide it and to protect it against environmental stress.

The following features are needed:

  • - Material : polyolefin, with UV absorber
  • - 40 mm diameter
  • - Flame retarding type

3.2.2 INTO THE WATER

Into the water, there is no need of cable protection, unless specific stress is expected.

3.2.3 ON-SHORE

The DC cable from main boxes or junction boxes to the inverters (marine cables) shall be placed into cable trays once onshore, either upon the ground or underground following the local standards.

The following features are recommended:

Type: Isolating cable trays, UNEX type

Material: Compliant with local standards and norms

Temperature range (shipping, stock, installation, use): from -20°C to +60°C

Impact resistance: 20J at -20°C

Load resistance: Compliant with local standards and norms, for outdoor use. For example, EN 61537 norm type I requires 1,5 meter space up to 40°C and 1 meter space up to 60°C (outdoor use).

Flame: Flame retarding in compliance with local standards and norms

GWIT: GWIT test local compliance

Shape: the bottom shall be flat; the structure shall be full, without any cutting ridges, and with a cowl. Joints shall adapt to possible dilatations.

Dimensions: it shall be oversized for filling by 30 %, and cable tray shall have the capacity to enable a 100 % filling.

3.3 SUBMARINE CABLE

We can provide cable references for this kind of equipment.

There are 2 cables of this type per junction box or main box (one + and one -).

Usually the following features are requested:

  • - 1000 V DC,
  • - Resistance to submersion : AD8 (french standard, meaning permanent submersion) or equivalent
  • - Flexible or stiff conductor, copper or aluminum, with a preference for aluminum
  • - Temperature max +90°C,
  • - Temperature min -50°C,
  • - Protection rating: IP 67 or equivalent
  • - Compliant with local standards and norms

We can help you to calculate the suitable cable section regarding the local standard for voltage drop (maximum admissible voltage drop).

3.4 STRING CABLE / CABLE FROM JUNCTION BOX TO MAIN BOX

These cables run from PV panels to junction boxes and from junction boxes to main boxes using the floats. They can be placed into a sheath if required.

Regarding the string cable especially, it is placed under the PV panel, between the PV panel and the float, but there is no duct to guide it. It can be placed into a sheath if required.

The following features are requested:

  • - 1000 V DC
  • - Flexible conductor, copper or aluminium
  • - Core temperature resistance: 120°C
  • - Resistance to water : AD7 (French standard, meaning possible immersion, partial or total, temporarily) or equivalent
  • - Compliant with local standards and norms

We can help you to calculate the suitable cable section regarding the local standard for voltage drop (maximum admissible voltage drop).

3.5 CABLE FROM PANEL TO PANEL

These cables run from PV panel to PV panel, thus they are not placed into sheath and just hang between the PV panels. Usually the provided length is just enough to enable the connection, thus it doesn’t hang loosely.

Selected PV panels shall be supplied with cables long enough: 1000 to 1200 mm, the total length (sum of the 2 cables) should be at least 2000 mm.

3.6 PV PANELS COOLING

We consider that the water temperature is often lower than the air temperature, so the temperature of the PV panels is lower too, and the yield better. It is natural refrigeration.

Tests are currently being run on its prototype at Piolenc to check the real effect of this natural cooling on electricity generation, but no definitive ratio or data can be yet published. Surely it improves the electricty generation but we have yet to determined on which scale.

Additionaly, it depends on many factors (air temperature on site, wind, sun irradiation that differs with the moment of the day…).

3.7 WHERE ARE THE POWER TRANSFORMATION DEVICES SUCH AS JUNCTION BOXES AND POWER CONDITIONERS INSTALLED?

Aluminium plates are fixed on the float, above which junction box is mounted. Power conditioner and power transformation device are installed on the bank.

3.8 ARE THERE ANY NEGATIVE IMPACTS REGARDING THE INSTALLATION OF PV PANELS ON WATER?

Panels and all electric devices contained are manufactured to be water-resistant so there are no negative impacts (However, as a matter of warranty, please request confirmation to each maker). Instead, because of the panel cable cooling, an increased power generation can be expected compared to ground and rooftop installations’ one.

3.9 WHAT KIND OF PANELS ARE COMPATIBLE WITH YOUR SOLUTION?

Regardless of the amount of power generation , 60 polycrystalline silicon-based cells panels will fit in most of the cases (standard compatible size: maximum length 1670mm (65,75in), minimum height 20mm (0,79in), frame thickness 25mm (0,98in) and upwards, cable length 900mm (35,43in) and upwards, compatible MC connector, maximum weight 25kg (55,12lb)). Regarding the panel you would like to use, please beforehand present us the panel data sheet to comfirm its compatibility.

4. ANCHORING SYSTEM

4.1 NEEDED DATA FOR PRELIMINARY DESIGN

Several data regarding the site and the solar islands are needed to be able to size the anchoring system.

  1. A design of the solar island is needed beforehand, in order to check the dimensions of the solar island.
  2. One of the most important data is the design wind speed.
  3. It can be calculated based on the local wind code. It can be found usually within the construction standards. Thus the detailed equations are needed as well as e map of basic wind speed. Otherwise, the client can provide its own data. Usually, 10-minutes mean wind speed with a return period of 50 years is requested. If not available, the client can provide its own estimation of the wind speed to use into the simulation, but in this case, we cannot warranty the reliability of the provided data.
  4. The depth of the reservoir and the water level variation are needed. It can be assumed for the preliminary anchoring study, but shall be better determined for further calculations.
  5. The map of the reservoir in order to define the shape of it and to check if the space needed for the anchoring system will fit the available space.

Finally, we can provide a preliminary anchoring design note, indicating the loads involved, the layout of the anchoring system and a preliminary part list.

NB: if additional loads are expected, such as snow loads or water stream loads, it should be mentioned to be taken into account for the calculation.

4.2 NEEDED DATA FOR PRELIMINARY DESIGN

In a second step, a detailed anchoring design can be carried out. At this stage of the project, more data are needed:

  • - Detailed map of the reservoir, ideally in .dwg or .dxf
  • - Ideally, a bathymetric map of the reservoir, or at least the bottom shape and the depth
  • - Composition of ground of the bottom or the banks (depending on the selected type of anchoring)

4.3 SOLAR ISLAND MOVEMENT

The anchoring system is designed for the worst scenario regarding water level. Thus when the water level is not in the worst case, and if the wind blows, the solar island may move on the water surface around the anchors places.

However, the movement amplitude is usually quite low because of the design (mooring line quite tightened). Thus there is no need to worry about this phenomenon, which is taken into account through the PV plant design process.

5. ENVIRONMENT AND AMENITY

5.1 IMPACT ON WATER DRINKABILITY

The HYDRELIO floats are made of HDPE that is normally neutral regarding the water.

It has been tested in the UK regarding the compliance with the standard BS 6920:2000 (odour & flavour test for non-metallic material for use with drinking water). The test was positive, and the conclusion is that the HYDRELIO is suitable for contact with cold but not hot water intended for human consumption.

There is no elution of toxic substance from the floats or effects on water quality (experiments have been conducted). One of the 3 largest water services company Thames Water (UK) carried out an experiment which approved the use of hydrelio technology on drinking water ponds. High-density polyethylene itself is a material that is also used for water pipes, so there are no concerns about the effects on drinking water. For all other equipments except the floats, please check with the manufacturer for the conditions of use.

5.2 IMPACT ON AQUATIC FAUNA AND FLORA

Several impacts are expected regarding the fauna and flora. The implementation of the solar islands on the water surface lead to a decrease of light going through the water. Therefore both light itself and light heath are decreasing, changing the balance of the reservoir fauna and flora for a new balance. Usually the water temperature decreases a bit, that is suitable for some species and not for others. The specific impacts on a specific site can be then identified only with the help of a local aquatic fauna and flora specialist.

The amplitude of these impacts is directly linked to the coverage ratio (area of the solar islands / water area).

Fortunately, these impacts remain highly moderated, and sometimes can even been considered as positive, in case of high eutrophication. For example, one of the positive aspects is the reduction of algae growth.

There is no impact on the natural environment. Instead, improvement of the water quality is expected due to the water temperature rise reduction and the water evaporation reduction. Also because of the shade effect, an inhibiting effect on the algae spring forth can be expected. However, if rare species are present in the pond, we recommend that you consult expert.

5.3 IMPACT ON WATER EVAPORATION

The act of covering the water surface by the solar islands helps to decrease the evaporation phenomenon. However, other phenomena can be responsible for water leaks (infiltration for example).

The reduction of evaporation is directly linked to the coverage ratio (especially the contact area, the area of the floats).

We can help you to estimate the evaporation reduction due to the implementation of the solar islands.

6. INSTALLATION ENVIRONMENT

6.1 CAN YOUR TECHNOLOGY BE INSTALLED IN A COLD REGION?

The installation of power generation equipment is not an issue, however the radiation amount might be a concern. In addition, even if frozen water has no impact on hydrelio, because a snowfall exceeding 70kg/m2 could cause the float to sink, it may become not eligible.

6.2 CAN IT EVEN BE INSTALLED IN A PROJECT WITH DEPTH ISSUES SUCH AS A DAM?

It depends on the actual spot conditions. If deep water currents are extremely violent, anchorage is complex and its cost is high. Ideally the depth of water should not exceed 10m and water current should not be over 1m/s.

6.3 WHAT IF THE POND IS EMPTY?

That is not a concern. The design is carried out to anticipate the water level variation. However, cases where the roughness of the pond bottom topography is extreme are not eligible. Beforehand, an investigation of the pond bottom topography is needed.

6.4 CAN IT BE INSTALLED IN A POND SUCH AS A REGULATION POND WHERE USUALLY THERE IS NO WATER?

The installation is not a matter of concern.

6.5 DO YOU PLAN TO INSTALL SOLAR PLANT ON SEA LEVEL?

That is not covered yet

6.6 WHAT ARE THE EFFECTS OF HURRICANES ON THE FLOATING PV PLANT?

The results of the wind tunnel testing had shown that hydrelio technology can withstand wind speed up to 58.3m/s (130mph) (regarding meteorological agencies definition of “extremely strong hurricane” wind speed range goes from 44m/s to 54m/s – 98mph to 130mph ). In July 2013, the grid connected Okegawa floating solar plant has been hit by a hurricane several times after installation, and there were no record of any damage.

6.7 HOW IS THE FLOATING PV PLANT FIXING SYSTEM DONE?

Because of the location the hydrelio is fixed using anchors. The standard process is a bottom anchoring system but dead weight anchorage and bank anchoring system are also feasible. The selection of the anchoring system is done taking into account, the drawdown, the depth of water, the ground among other parameters.

7. CONSTRUCTION

7.1 HOW MUCH TIME DOES IT TAKE TO CARRY OUT THE INSTALLATION OF FLOATS?

For the assembly of floats and panels mounting there is no need of any special tool, as a result the installation work of the hydrelio technology is quick and simple. In actual practice, assembly of approximately 2 kwp per hour with 2 workers (8 set) is possible.

7.2 IS IT POSSIBLE TO RECEIVE TRAINING FOR CONSTRUCTION WORK?

The dispatch of a consultant and on-site construction supervisor training is possible. Regarding the scale of the project, the number of days of free of charge dispatch may vary.

7.3 HOW MUCH SPACE ON LAND IS NEEDED TO CARRY OUT THE CONSTRUCTION WORK?

A place at about the same height than the water surface, a space of one row length x 5 meters (x 16,4ft) large is the minimal condition. If there is not enough place the preparation of a temporary platform is needed. In actual practice, we can provide consulting to determine construction location, platform position, slope and raw materials. Also, it’s essential to ensure a material storehouse near the pond.

8. DESIGN

8.1 HOW IS THE FLOATING PV PLANT FIXING SYSTEM DONE?

Floats are fixed using anchors. Standard process is a pond bottom anchorage but there is situation where bank anchorage is possible. Anchorage process is determine taking into account site actual conditions (water level variation, depth of water, ground).

8.2 HOW DO YOU MAKE A GOOD DESIGN?

Floating island design and anchoring design are Ciel & Terre fundamental duty. On the other side, the wiring design is the responsibility of the electric power company.

8.3 WHAT KIND OF INFORMATION IS REQUIRED FOR THE FLOATING ISLAND AND ANCHORING SYSTEM?

Our company offer design based on the fill of a “Checklist” of site actual conditions. For a concrete design, please provide us with as much information as possible. However, as long as we know the location of the pond a preliminary design can be made. For a detailed anchoring design pond measurement data required.

8.4 IF THE WATER LEVEL DECLINES AND THE ANCHOR CABLES LOOSEN, HOW WIDE IS THE FLOATING ISLAND MOVEMENT AREA?

Up to 10m (32,81ft) water level fluctuations can be coped with depending on the conditions. Even with water level fluctuation, the design of the floating island is made to restrain to a minimum its movement area. Futhermore, we are able to calculate the maximum movement of the island.

8.5 WHAT IS THE WEIGHT LOAD OF THE FLOAT?

Assuming a 2 racks of main floats and 3 racks of secondary floats configuration (including panels), the entire system is designed to withstand up to a weight of 243kg (535,72lb). However, the usual island withstand weight load is higher because, the more the island is large, the more buoyancy is high and so is the withstand weight load.

8.6 WHAT IS THE WEIGHT LOAD OF THE FLOAT?

It digs in 5 to 8 cm (1,97 to 3,15 in).

9. MAINTENANCE AND WARRANTY

9.1 WHAT KIND OF MAINTENANCE IS REQUIRED?

At least once year, and also after hurricane and storm an inspection work is required. On this occasion, float junctions, float’s caps loss and panel fixation condition have to be checked. Ideally, simultaneously underwater parts (anchoring cables and shackles) inspection works too is recommended.

9.2 WHAT ABOUT TERM OF WARRANTY?

Manufacturer warranty for the floats is 5 years. A maximum 15 years warranty extension can be offered with additional fee. However, in cases of a warranty extension, maintenance to adapt to the recommendation of the manufacturer has to be performed, and results need to be reported. Furthermore, we propose chargeable maintenance program from the manufacturer.

10. ACHIEVEMENTS

10.1 IS THERE A POPULAR TYPE OF FLOATING INSTALLATION TYPE OF MEGA SOLAR PLANT IN THE WORLD?

We achieved some projects in France and in the UK, but large scale floating solar power plant crossing over 1MW were only achieved in Japan so far. There is countless pond suitable for floating solar power plant in Japan which mean a great potential.

10.2 COULD YOU TELL ME MORE ABOUT CIEL & TERRE MEGA SOLAR POWER PLANT MODEL ACTUAL RESULTS?

An operational floating solar power plant prototype in France 3 years ago, a 200kW floating solar power plant in the UK, and 4 actually operational power plant in Japan for a total of 3.8 MW were achieved.

10.3 IS IT CIEL & TERRE WHO CAN PROVIDE FLOATS TO CARRY OUT FLOATING INSTALLATION TYPES OF MEGA SOLAR POWER PLANTS?

In homeland or overseas, there are competitors. However, at that time Ciel & Terre only achieved mega solar power plant (more than 1MW). Furthermore, at the moment Ciel & Terre only has more than 3 years of track records and patented floats.